I think there is a clash of cultures here. In the UK both the employer and employee has duties to ensure a safe working environment. If I went stateside and was asked/told to use a machine that would put me on the top of the chip pile I would happily walk away.
I don't think there would be many tree care companies in the UK that would allow 1 or 2 operators to use these machines manually. The risk to the owners freedom would be to high. If they are then i would recon that it would be the most experienced operators who would be working them, not unskilled immigrants.
Any machinery I have seen coming from the states has had safety features retro fitted to EU standards. (CE marked) This would say to me that we are more protected this side of the pond regarding safety regulations. Of the limited portrayal of the US logging scene and working conditions in general I think you guys are a few decades behind. That's not to offend the skill and hard graft, just the systems in place.
Over the past few years I think our culture is getting even better regarding the mindset of the workers attending training, sure there are older workers who scoff at alot of the new regulations but the younger ones are starting to take it more seriously.
Our industry guidelines advise against lone working. I wouldn't think there are many sole traders with monster chippers here.
As for the climbers comments. I don't think you can compare the two. The risk assessment would determine if you work from the ground, use machinery(MEWP) or then if appropriate climb. If climbing then the proper gear and climber for rescue. (Which is the same if using a MEWP, need someone at the bottom to bring cage down)
Chippers I have been trained on and used have enough safety features to provide a safe working environment and system of planned work. IMO.
As long as we all go home safely and don't ruin peoples chip piles, sobering to remember we are only here once.
Hope that makes sense!